In a superconductor, the ratio of the carrier density, n, to their effective mass, m * , is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two dimensional systems, this ratio n/m * (∼ 1/λ 2 L ) determines the effective Fermi temperature, TF . We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1−xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 K. This structure may arise from quantum fluctuations associated with a quantum critical point (QCP). The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover towards the Bose-Einstein condensate limit driven by quantum criticality.In two families of high temperature superconductors, cuprates and iron-pnictides, superconductivity emerges in close proximity to an antiferromagnetically ordered state, and the critical temperature T c has a dome shaped dependence on doping or pressure [1][2][3]. What happens inside this superconducting dome is still a matter of debate [3][4][5]. In particular, elucidating whether a quantum critical point (QCP) is hidden inside it (Figs. 1A and B) may be key to understanding high-T c superconductivity [4,5]. A QCP marks the position of a quantum phase transition (QPT), a zero temperature phase transition driven by quantum fluctuations [7].The London penetration depth λ L is a property that may be measured at low temperature in the superconducting state to probe the electronic structure of the material, and look for signatures of a QCP. The absolute value of λ L in the zero-temperature limit immediately gives the superfluid density λ −2which is a direct probe of the superconducting state; here m * i and n i are the effective mass and concentration of the superconducting carriers in band i, respectively [8]. Measurements on high-quality crystals are necessary because impurities and inhomogeneity may otherwise wipe out the signatures of the QPT. Another advantage of this approach is that it does not require the application of a strong magnetic field, which may induce a different QCP or shift the zero-field QCP [9].BaFe 2 (As 1−x P x ) 2 is a particularly suitable system for penetration depth measurements as, in contrast to most other Fe-based superconductors, very clean [10] and homogeneous crystals of the whole composition series can be grown [11]. In this system, the isovalent substitution of P for As in the parent compound BaFe 2 As 2 offers an elegant way to suppress magnetism and induce superconductivity [11]. Non-Fermi liquid properties are apparent in the normal state above the superconducting dome ( Fig. 2A) [11,12] and de Haas-van Alphen (dHvA) oscillations [10] have been observed over a wide x range including the superconducting compositions, giving detailed information on the electronic structure. Because P and As are isoelectric, the system remains compensated for all values of x (i.e., volumes of the electron and hole Fermi surfaces...

Among the iron-based pnictide superconductors the material KFe2As2 is unusual in that its Fermi surface does not consist of quasi-nested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio > 1200. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe2As2 is an unconventional superconductor with strong electron correlations.

High-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting behaviors. In LiFeAs the low-temperature λ(T) shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a T-linear dependence of superfluid density infinity λ(-2), indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of ~1.33 Å.

When a second-order magnetic phase transition is tuned to zero temperature by a nonthermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these "quantum critical" superconductors it has been widely reported that the normal-state properties above the superconducting transition temperature T c often exhibit anomalous non-Fermi liquid behaviors and enhanced electron correlations. However, the effect of these strong critical fluctuations on the superconducting condensate below T c is less well established. Here we report measurements of the magnetic penetration depth in heavy-fermion, iron-pnictide, and organic superconductors located close to antiferromagnetic quantum critical points, showing that the superfluid density in these nodal superconductors universally exhibits, unlike the expected T-linear dependence, an anomalous 3/2 power-law temperature dependence over a wide temperature range. We propose that this noninteger power law can be explained if a strong renormalization of effective Fermi velocity due to quantum fluctuations occurs only for momenta k close to the nodes in the superconducting energy gap Δ(k). We suggest that such "nodal criticality" may have an impact on low-energy properties of quantum critical superconductors. T he physics of materials located close to a quantum critical point (QCP) are an important issue because the critical fluctuations associated with this point may produce unconventional high-temperature superconductivity (1, 2). Quantum oscillations (3, 4) and specific heat measurements (5) have shown that, in some systems, as the material is tuned toward the QCP by controlling an external parameter such as doping, pressure, or magnetic field, the effective mass strongly increases due to enhanced correlation effects. Along with this the temperature dependence of the resistivity shows a strong deviation from the standard AT 2 dependence in the Fermi liquid (FL) theory of metals and often shows an anomalous T-linear behavior that corresponds to the A coefficient diverging as zero temperature is approached.Although there are many studies of non-FL behavior in the normal metallic state (1, 2), relatively little is known about how the QCP affects the superconducting properties below the critical temperature T c . The superconducting dome often develops around the putative QCP so that when the temperature is lowered below T c , the superconducting order parameter starts to develop and the Fermi surface becomes gapped. It is therefore natural to consider that the low-energy quantum critical fluctuations are quenched by the formation of the superconducting gap Δ, which means that the system avoids the anomalous singularities associated with the QCP. Perhaps because of this reasoning the superconducting properties are usually analyzed by the conventional theory without including temperature/field-dependent renormalization effects resulting from the proximity to the QCP. For example, in refs. 6 and 7 the NMR relaxat...